Two major objectives of the present research plan are to study the: (a) neuronal circuitry and (b) synaptic mechanisms that contribute to the response and receptive field characteristics of cat spinomesencephalic tract (SMT) cells. These objectives will be addressed by three types of experiments. (1) The intracellular injection of horseradish peroxidase will be used to study the cell morphologies and dendritic arborizations of functionally characterized SMT cells. (2) Electrical stimulation with electrodes positioned at different brainstem levels will be used to map the distribution of sites producing excitatory and/or inhibitory effects on different classes of SMT cells. Spinal lesions will also be used to localize the spinal trajectory of descending pathways responsible for these effects and to determine the extent to which the complex receptive fields of SMT cells are due to the descending influence of supraspinal structures and/or local circuitry within the spinal cord. (3) Using intracellular recording techniques the inhibitory effects of peripheral stimulation on SMT cells will be studied to determine if these effects are due to pre- and/or postsynaptic mechanisms. The third objective of the research plan is to further evaluate the SMT projection to different midbrain regions. This will be studied by systematically mapping the termination site(s) of SMT axons using the technique of antidromic activation. Antidromic threshold maps will extend into the medial and lateral thalamus in an effort to study the collateralization of cat SMT axons. Previous studies related to the cat SMT have shown this pathway to be made up of a heterogenous population of neurons with varied receptive field and response properties, spinal origins, axonal trajectories and sites of termination. The potential involvement of this pathway in nociception is supported by the demonstration that the majority of SMT cells respond to noxious mechanical and thermal stimuli. Additional studies related to the anatomy and physiology of this pathway will contribute to our understanding of substrates potentially responsible for the multidimensional aspects of pain. In addition to providing a better understanding of the anatomy and physiology of SMT cells, the specific aims of the research outlined above are intended to extend our basic knowledge of sensory mechanisms in the spinal cord and particularly that related to nociception.